As for a representative film forming method for forming a solid thin film on a surface of a substrate such as a semiconductor wafer or the like, there is known a CVD (chemical vapor deposition) method. When a film is formed by using the CVD method, a source needs to be activated by applying energy to a source gas. Accordingly, there has been employed a CVD method for supplying thermal energy to a source gas through a substrate heated by a heater provided at a mounting table for mounting thereon the substrate or a plasma CVD method for supplying energy of a plasma generated in a space above a substrate by introducing a source gas into the atmosphere thereof.
A film forming apparatus for manufacturing an advanced very large scale integrated circuit needs to have a performance (a step coverage performance) of forming a high-quality thin film of a uniform thickness along surfaces of holes/grooves previously formed on a surface of a semiconductor wafer with a diameter/width of tens of nanometers.
In order to obtain a high step coverage performance, a surface reaction needs to take place by activating a source gas on an uppermost surface of a substrate, not by activating it in a gas phase space above the substrate. However, in the CVD method for forming a film by continuously supplying a source gas, a reactant gas and energy, certain source gases may cause a gas phase reaction by an excessive activation thereof in a gas phase. Since the gas phase reaction greatly deteriorates the step coverage performance, there arises a need to suppress the gas phase reaction and facilitate the surface reaction in order to maintain the high step coverage performance.
As for another method for forming a thin film on a surface of a substrate, there is known an atomic layer deposition (ALD) method. With the ALD method, a thin film having high step coverage can be formed on a substrate disposed inside a vacuum chamber by repetitively performing a film forming process and a purge process. In the ALD film forming process, a reaction is carried out by supplying energy to a monomolecular or a multimolecular adsorption layer adsorbed on a surface, the adsorption layer being formed of molecules of a source compound. In the purge process, the atmosphere inside the vacuum chamber is substituted.
The ALD method for forming a film while suppressing a gas phase reaction was suggested in 1977 by Suntola et al. (see U.S. Pat. No. 4,058,430). The method is performed by alternately supplying a source gas and a reactant gas to a substrate at different timings, as shown in FIG. 24, and then removing a residual source gas and a by-product gas of a previous cycle remaining in a gas phase with a non-reactive purge gas before supplying the source gas and the reactant gas again. The gas phase reaction can be suppressed by repeating those cycles. Further, the high step coverage performance can be maintained by restricting the reaction to take place at the uppermost surface of the substrate. There are plenty of reports on the ALD method (see, e.g., R. L. Puurunen, “Surface chemistry of atomic layer deposition: A case study for the triethylaluminum/waterprocess”, Journal of Applied Physics, APPLIED PHYSICS REVIEW, vol. 97, p 121301 (2005)).
In an initial ALD method, although the source gas and the reactant gas are separately provided as shown in FIG. 24, the energy (heat) is constantly supplied. This is because the initial ALD method supplies the thermal energy to a surface of a substrate via the substrate by heating the entire substrate as in the thermal CVD method and, therefore, a time responsiveness in controlling an on/off of energy supply becomes poor (see, e.g., U.S. Pat. No. 4,389,973). Such an ALD method is referred to as “thermal ALD method”. In this method, since the energy is constantly supplied during a source gas supply process, parts of the source gas may ever cause an self-pyrolysis reaction in a gas phase by receiving the thermal energy transferred in the gas phase from the substrate, which leads to a deterioration of the step coverage performance.
Moreover, since the entire substrate is heated constantly during the processing, a solid layer that has been formed by a previous process may be deteriorated by the heat.
To solve those drawbacks, Sherman et al. has suggested an ALD method for supplying energy by radicals generated by an RF power supply (see U.S. Pat. No. 5,916,365). Further, Chiang et al. has suggested a method for supplying energy by radicals and ions generated from a plasma (see U.S. Pat. No. 6,416,822). In these methods, the energy is supplied not by heat but by chemically active species (radicals, ions or combination thereof) generated from the RF power supply, so that an ON/OFF of energy supply can be controlled with fine time-responsiveness. Such an ALD method is referred to as “a plasma-assisted ALD method”.
In the plasma-assisted ALD method, a source gas supplying process and an energy supplying process can be carried out at different timings. Therefore, it is possible to prevent a self-pyrolysis reaction of the source gas from taking place during the source gas supplying process, the self-pyrolysis reaction being caused by the continuous supply of thermal energy. Further, since such a method is not a method for supplying energy through a substrate being continuously heated, it is possible to avoid the problem of deteriorating the previously formed solid layer with the heat.
However, the method using as an energy source radicals or ions generated from a plasma has new problems to be described as follows.
Firstly, the excessively high energy of the active species (radicals, ions and electrons) generated from a plasma inflicts a serious physical damage or causes a chemical deterioration on a base layer of a substrate where a film will be formed (see, e.g., A. Grill et al, “Hydrogen plasma effects on ultra low-k porous SiCOH dielectric”, Journal of Applied Physics, vol. 98, p 074502 (2005)).
Secondly, the active species also collide against not only the substrate but also an inner surface of an apparatus in contact with the plasma, thereby causing a physical sputtering, which in turn result in impurity incorporation into the surface of the substrate.
Thirdly, since the energy is also supplied by the active species to side chain groups contained in the source gas that are desirably to be removed by the reaction, the side chain groups may be incorporated as undesired impurities into the film.
Fourthly, a potential gradient generated inside the apparatus electrically may destroy fine integrated circuits formed on the substrate.
Fifthly, high energy ultraviolet rays generated from the plasma may deteriorate the base layer of the substrate.
As long as the energy is supplied by using the plasma, the aforementioned problems may be only partially, but not completely, avoided.
In order to avoid those problems generated in supplying energy by the plasma, Chiang et al. has suggested a method for supplying energy by light (see, U.S. Pat. No. 6,878,402). In case the energy is supplied by irradiating light on a surface of a substrate, a window for transmitting the light needs to be provided above the substrate. Since, however, a surface of the window becomes dirty during a film forming process, the light is reflected or absorbed and, thus, an intensity of the light reaching the substrate decreases. Moreover, in case the surface of a processing target substrate is made of a metal, the light is reflected on the surface of the substrate, which hinders the energy supply for the reaction.